Data Series 282
U.S. GEOLOGICAL SURVEY
Data Series 282
Three different types of optical sensors were used to monitor SSC during WY 2005. The first type of sensor is manufactured by D & A Instrument Company and is a cylinder approximately 7 inches (in.) long and 1 in. in diameter with an optical window at one end, a cable connection at the other end, and an encased circuit board. A high-intensity infrared-emitting diode produces a beam through the optical window that is scattered, or reflected, by particles that are about 0.2–12 in. in front of the window. A detector (four photodiodes) receives backscatter from a field of 140–165 degrees which is converted to a voltage output and recorded on a separate data logger. The second type of sensor, manufactured by Forest Technology Systems (FTS), is self-cleaning and differs from the D & A Instrument Company sensor in that it measures the intensity of light scattered at 90 degrees between a laser diode and a high-sensitivity silicon photodiode detector. The output, in nephelometric turbidity units (NTU), is recorded on a separate data logger. The third type of sensor, versions of which are used by both Hydrolab and YSI instruments, measures the intensity of light scattered at 90 degrees between a light-emitting diode and a high-sensitivity photodiode detector, and the output (NTU) is processed by internal software. The Hydrolab and YSI instruments (sondes) are self-contained, including a power source, data logger, and the capability of supporting additional sensors.
Optical sensors were positioned in the water column by using polyvinyl chloride (PVC) pipe carriages coated with an antifoulant paint to impede biological growth. Carriages were designed to align with the direction of flow and to ride along a stainless steel suspension line attached to an anchor weight, which allowed sensors to be easily raised and lowered for servicing (fig. 2). The plane of the optical window maintained a position parallel to the direction of flow as the carriage aligned itself with the changing direction of flow. Optical sensor depths in the water column are listed in table 1.
Biological growth (fouling) interferes with the collection of accurate optical-sensor data. Fouling generally was greatest on the sensor closest to the water surface; however, at shallower sites where the upper sensor was set 10 ft above the lower sensor, fouling was similar on both sensors. Self-cleaning optical sensors were used where conditions allowed. Because of the difficulty in servicing some of the monitoring stations, sensors were cleaned manually every 1–5 (usually 3) weeks. Fouling would begin to affect sensor output from 2 days to several weeks after cleaning, depending on the level of biological activity in the bay. Generally, biological fouling was greatest during spring and summer.
On-site checks of sensor accuracy were performed using turbidity solutions prepared from a 4,000-NTU formazin standard. Formazin is an aqueous suspension of an insoluble polymer and is the primary turbidity standard (Greenberg and others, 1992). The turbidity solutions were prepared by diluting a 4,000-NTU stock standard with de-ionized water in a clean, sealable container. Prepared solutions ranged from 20 to 180 NTU. Prepared solutions were checked with a Hach Drel 2000 Spectrophotometer for accuracy. At the field site, the cleaned sensors were immersed in the solution and the output was recorded on the station log. Monitoring a period of sensor performance in a known standard helps to identify output drift or sensor malfunction.
Data acquisition was controlled by electronic data loggers. The logger used with the D & A Instrument Company sensor was programmed to power the optical sensor every 15 minutes, collect data each second for 1 minute, then average and store the output voltage for that 1-minute period. The Hydrolab, YSI and FTS data loggers collect instantaneous values every 15 minutes. Power was supplied by 12-volt batteries.
SSC data were collected in Suisun Bay at Mallard Island and at Benicia Bridge (fig. 1, table 1). Optical sensors were installed at the DWR Mallard Island Compliance Monitoring Station on February 8, 1994. Optical sensors were positioned to coincide with DWR near-bottom electrical conductance and temperature sensors and the near-surface pump intake. The pump intake is attached to a float and draws water from about 3 ft below the surface. The near-surface optical sensor is attached to a separate float and positioned at the same depth as the pump intake.
Optical sensors were installed at Pier 7 on the Benicia Bridge on March 15, 1996. The Benicia Bridge site was shut down August 7, 1998, for seismic retrofitting of the bridge and was reestablished with sondes equiped with optical, conductance, and temperature sensors on May 1, 2001. A monitoring site at the Martinez Marina fishing pier was discontinued in WY 1996 because data from the Benicia Bridge site were considered more representative of SSC in the Carquinez Strait area of Suisun Bay (Buchanan and Schoellhamer, 1998).
SSC data were collected in Carquinez Strait at Carquinez Bridge, Napa River at Mare Island Causeway, and San Pablo Bay at Channel Marker 1 (fig. 1, table 1). Sondes with optical, conductance, and temperature sensors were installed at the center pier structure at Carquinez Bridge on April 21, 1998. Optical sensors were installed off a catwalk beneath Mare Island Causeway on October 1, 1998. A sonde with optical, conductance, and temperature sensors was installed at USCG Channel Marker 1 on October 7, 2003. A monitoring site at USCG Channel Marker 9 was discontinued in WY 2003 because data from the USCG Channel Marker 1 site were considered less affected by the processes that occur at the mouth of the Petaluma River (Ganju and others, 2004).
SSC data were collected in San Pablo Strait at Point San Pablo and San Francisco Bay at Alcatraz Island (fig. 1, table 1). Optical sensors were installed in San Pablo Strait at the northern end of the Richmond Terminal no. 4 pier on the western side of Point San Pablo on December 1, 1992. The station at Point San Pablo was shut down on January 2, 2001, and reestablished on December 11, 2001, at a pier-adjacent structure approximately 25 ft from the previous deployment site. A sonde with optical, conductance, and temperature sensors was installed on the northeast side of Alcatraz Island on November 6, 2003. A monitoring station at San Francisco Bay at Pier 24 was discontinued on January 3, 2002. The USGS assumed operation of the stations at Point San Pablo and Pier 24 from DWR in October 1989, although the collection of conductivity and temperature data continued to be cooperatively funded by DWR and the USGS. A monitoring station at the south tower of the Golden Gate Bridge was operational during water years 1996 and 1997. Conductivity and temperature data collected at Point San Pablo and Pier 24 prior to October 1, 1989, can be obtained from DWR.
SSC data were collected in South San Francisco Bay at San Mateo Bridge, Dumbarton Bridge, and USCG Channel Marker 17 (fig. 1, table 1). Optical sensors were installed at Pier 20 on the San Mateo Bridge, on the east side of the ship channel, on December 23, 1991. In addition to SSC, specific conductance and temperature were monitored at near-bottom and near-surface depths at San Mateo Bridge. The USGS assumed operation of this station from DWR in October 1989, although the collection of specific conductance and temperature data continued to be cooperatively funded by DWR and USGS. Specific conductance and temperature data collected at San Mateo Bridge prior to October 1, 1989, can be obtained from DWR. Optical sensors were installed at Pier 23 on the Dumbarton Bridge on the west side of the ship channel on October 21, 1992. Optical sensors were installed at USCG Channel Marker 17 on February 26, 1992.
Water samples, used to calibrate the output of the optical sensors to SSC, were collected by using a horizontally positioned Van Dorn sampler before and after the sensors were cleaned. The Van Dorn sampler is a plastic tube with rubber stoppers at each end that snap shut when triggered by a small weight dropped down a suspension cable. The Van Dorn sampler was lowered to the depth of the sensor by a reel and crane assembly and triggered while the sensor was collecting data. After collection, the water sample was marked for identification and placed in a clean, 1-liter plastic bottle for transport. The SSC of water samples collected with a Van Dorn sampler and a P-72 point sampler, used until WY 1994, were virtually identical (Buchanan and others, 1996).
Samples were sent to the USGS Sediment Laboratory in Marina, California, for analysis of SSC. Suspended sediment includes all particles in the sample that do not pass through a 0.45-micrometer membrane filter. The analytical method used to quantify concentrations of suspended solid-phase material was consistent from 1992 through the present study; however, the nomenclature used to describe sediment data was changed (Gray and others, 2000). Suspended-sediment concentrations were referred to as suspended-solids concentrations in previous reports (Buchanan and Schoellhamer, 1995, 1996, 1998, 1999; Buchanan and others, 1996; Buchanan and Ruhl, 2000, 2001). Water samples collected for this study were analyzed for SSC, in milligrams per liter (mg/L), by filtering samples through a pre-weighed, tared, 0.45-micrometer membrane filter. The filtrate was rinsed with de-ionized water to remove salts, and the insoluble material and filter were dried at 103°C and weighed (Fishman and Friedman, 1989).
Data loggers stored the optical-sensor output at 15-minute intervals (96 data points per day). Recorded data were downloaded from the data loggers onto either a storage module or laptop computer during site visits. Raw data from the storage modules or laptop computer were loaded into the USGS Automated Data-Processing System (ADAPS).
The time-series data were retrieved from ADAPS and processed to remove invalid data. Invalid data included rapidly increasing voltage outputs and unusually high voltage outputs of short duration. As biological growth accumulated on the optical sensors, the voltage output of the sensors increased (except for the Hydrolab’s optical sensor output, which decreased). An example time-series of raw and processed optical sensor data is presented in figure 3. After sensors were cleaned, sensor output immediately decreased (fig. 3A: July 20, August 8, 29, and September 19; dates represented by vertical dashed lines). Efforts to correct for biofouling proved to be unsuccessful because the signal often was highly variable. Thus, data affected by biofouling often were unusable and were removed from the record (fig. 3B). Identifying the point at which fouling begins to affect optical sensor data is somewhat subjective. Indicators, such as an elevated baseline, increasingly variable signal, and comparisons with the other sensor at the site, are used to help define the point at which fouling begins to take place. Spikes in the data, which are anomalously high voltages probably caused by debris temporarily wrapped around the sensor or by large marine organisms (fish, crabs) on or near the sensor, also were removed from the raw data record (fig. 3B). Sometimes, incomplete cleaning of a sensor would cause a small, constant shift in sensor output that could be corrected by using water-sample data that had been collected for calibration of the sensors.
U.S. Department of the Interior | U.S. Geological Survey
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